You also have to look at the study methods. The average age of participants was in their 50s, from what I remember, many with comorbidities. So when they looked at the bell shaped curve they determined that the 2nd dose was important for the average study participant. But that is not necessarily true of a healthy 20 year old, or even an unhealthy 90 year old. Maybe a single dose would be fine for a 20 year old, and maybe two doses isn’t enough for a 90 year old (which is largely why we are getting boosters). The same rationale applies for the decision whether to get a booster. It may not be absolutely necessary for an otherwise healthy 20 year old…but absolutely necessary for a 90 year old. But we are very broadly applying recommendations for everyone out of simplicity, to prevent confusion, and to be conservative.